This invention relates to communication systems and devices and more particularly to systems and devices that keep track of potential receive window positions to prevent the receive window from moving from the proper potential positions, thereby reducing the probability of incorrect positions.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focusses on GSM/EDGE systems for economy of explanation, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
FIG. 1 depicts a mobile radio cellular telecommunication system 10, which may be, for example, a GSM or a GSM/EDGE communication system. Radio network controllers (RNCs) 12, 14 control various radio network functions including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs calls to and from a mobile station (MS), or remote terminal or user equipment (UE), via the appropriate base station(s) (BSs), which communicate with each other through downlink (DL) (i.e., base-to-mobile or forward) and uplink (UL) (i.e., mobile-to-base or reverse) channels. RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26. Each BS, which is a Node B in 3G vocabulary, serves a geographical area that can be divided into one or more cell(s). BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes like a mobile switching center (not shown) and/or a packet radio service node (not shown). In FIG. 1, MSs 28, 30 are shown communicating with plural base stations: MS 28 communicates with BSs 16, 18, 20, and MS 30 communicates with BSs 20, 22. A control link between RNCs 12, 14 permits diversity communications to/from MS 30 via BSs 20, 22.
A multimedia broadcast and multicast service (MBMS) for GSM and GSM/EDGE communication systems is being standardized by 3GPP. MBMS is described in 3GPP Technical Specification TS 43.246 ver. 6.2.0 Technical Specification Group GSM/EDGE Radio Access Network; Multimedia Broadcast Multicast Service (MBMS) in the GERAN; Stage 2 (Release 6) (January 2005), among other places.
MBMS offers high-speed and high-quality broadcast, or multicast, transmission to mobile stations (UEs). An important feature of MBMS will be to make the use of radio resources efficient by allowing point-to-multipoint (p-t-m) transmission over the air, which is to say that many terminals will be able to listen to the same transmission. This has motivated the introduction of a third radio link control (RLC) mode, the non-persistent mode, in addition to the other two RLC modes, unacknowledged mode and acknowledged mode. These three RLC modes are described in chapter 9 of 3GPP TS 44.060 General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol (Release 6) V6.10.0, among other places.
The non-persistent transmission mode resembles the acknowledged and unacknowledged DL transmission modes of GSM/EDGE packet data (GPRS and enhanced GPRS (EGPRS)). In acknowledged mode, the network can at any time request the terminal to send an acknowledgement/negative acknowledgement (ACK/NACK) feedback message indicating which Radio Link Control (RLC) blocks have or have not been correctly received so far, and all blocks must be re-transmitted until they have been reported as successfully received by the terminal. In unacknowledged mode, no ACK/NACK feedback is possible and only one transmission attempt is permitted for each RLC block; if a block is not correctly received after the first attempt, it is considered lost by the terminal.
In the non-persistent mode, RLC blocks may be lost, like GSM/EDGE unacknowledged mode, but also multiple transmissions of each RLC block are possible and the network may, if it wishes, poll the terminal for ACK/NACK feedback, like GSM/EDGE acknowledged mode. The freedom to poll or not to poll terminals, and to make re-transmissions even without polling, makes the non-persistent mode very useful for the p-t-m transmission of MBMS. Nevertheless, the non-persistent mode is not without problems.
Data to be transmitted in a communication system such as that depicted in FIG. 1 is split into RLC blocks for transmission over the air. Each RLC block has a header that contains, among other things, a block sequence number (BSN). The BSN in each RLC block is encoded with eleven bits, which is to say that 2048 different BSN values can be encoded. Many kinds of data, particularly MBMS data, require RLC blocks that number more than 2048. In order to handle block numbers outside the range 0-2047, the block number N is encoded as before modulo-2048. In order to make a block number N unambiguously derivable from a block sequence number BSN, different rules for the three RLC transmission modes are standardized.
Each RLC block header also includes a cyclic redundancy check (CRC) value, which in both GPRS and EGPRS systems is short, only eight bits. An 8-bit CRC value means that a header error has a probability of 1/256 of not being detected, assuming evenly distributed bit errors in the decoded block. An RLC block with an undetected header error is likely to have an incorrect BSN, which may cause a false (and often large) increase of N0 in non-persistent RLC mode, as well as in unacknowledged RLC mode. N0 is a number that is simply one more than the largest block number N yet received and that is used in determining the position and size of a “receive window” for deriving block numbers from block sequence numbers. This can lead to misinterpretation of the block numbers of many subsequent (correctly received) blocks.